Genetic influences on childhood IQ in 5- and 7-year-old Dutch twins

To study changes in the genetic architecture of cognitive functioning as children grow up, we measured intelligence in a sample of young Dutch twins at ages 5 and 7 years. By applying a developmental model to the data, we could examine the continuity and change in genetic and environmental influences from age 5 to age 7. Results show that genetic influences persist from age 5 to age 7 and greatly gain in relative importance during this period. There is no evidence of new genetic influences being expressed at age 7. The influence of genetic factors on intelligence at age 5 is relatively small, accounting for 27% of the total phenotypic variance. At age 7, genetic factors explain 62% of the variance in IQ scores. This increase in heritability is due both to an increase in the total genetic variance and to a decrease in the amount of variance explained by shared environmental factors. The shared family background influences IQ in early childhood, and these effects also persist until age 7, but their relative importance becomes smaller. At age 5, the shared family environment accounts for half of the variance in IQ. At age 7, the influence of the family environment decreases to 10%. Unique environment acts age specifically and accounts for roughly a quarter of the total variance in IQ both at ages 5 and 7.